In the physical metal deposition step of wafer processing, increased plasma density improves the performance of the processing tool and extends the technology to the processing of wafers with finer features. Typically, a dense plasma is inductively generated with an antenna, either internal or external to the processing volume. The increased plasma density results in increased fractional ionization of metal atoms, sputtered from a sputtering target having a sputtering surface in the processing volume, for deposition onto a substrate wafer being processed in the volume. This, combined with RF bias on the wafer leads to improved coverage of the features on the wafer surface and reduces their closing-off. Likewise, during a separate stage of the Ionized Physical Vapor Deposition (IPVD) process, the soft-etch stage, an auxiliary plasma source is necessary to provide a uniform and dense plasma at the wafer level, so as to enable a uniform etch at an acceptable rate, but at a reduced DC bias voltage level. The plasma uniformity and low bias voltage are necessary to eliminate plasma damage. Finally, during the pre-clean stage of wafer processing, it is again desirable to perform a wafer soft-etch. As in the case of the soft-etch process, plasma density and uniformity are critical for process throughput and avoiding plasma damage.
One example of a prior art to IPVD uses an RF antenna mounted inside the process volume and operated at a frequency of 450 kHz so as to reduce the RF voltages on the antenna. A disadvantage of this approach has been that the RF antenna is consumable in that, due to the large RF voltages of at least several hundred Volts that develop on the antenna, the plasma sputters material from the antenna, eventually necessitating the replacement of the antenna.
Another disadvantage of the internal antenna approach has been that antenna cooling has been required, which complicates and the operation of the processing module. For example, to prevent overheating of the antenna from the plasma heat load, the antenna has had to be water-cooled. During operation, care has had to be taken not to punch through to the water-cooling channel, either by sputtering through the walls or burning through by arcing.
An alternate prior art approach to the internal antenna in IPVD is that adopted by Drewery et al. in U.S. Pat. No. 6,080,287. In this approach, the RF antenna is external to the processing chamber, and is situated in air, with the PF field from the antenna penetrating into the chamber through a dielectric window. To preserve the transparency of the dielectric window to the RF, the window is shielded from metal ions in the plasma in a manner that would still provide RF transparency by the introduction of a deposition shield. The shield is designed in a manner to be opaque to most of the metal ions but transparent to RF. As discussed by Drewery et al., this is accomplished by introducing a shield of electrically conductive material with long slots generally perpendicular to the direction of the RF antenna segments. The slot cross-section is designed to block the vast majority of metal flux, while permitting the passage of RF flux. This approach removes many of the problems of the internal antenna design.
Used with an external antenna, the deposition shield must accomplish two counteracting goals: RF transparency and opacity to metal transport. These goals are not easily accomplished, as one is usually achieved at the expense of the other. The deposition shield is either made of one piece with a complex slot shape (a slot of chevron shaped cross-section is one such example) or a two-piece assembly with overlapping slats must be produced. The single shield has a higher RF transparency but is more complex to manufacture. The two-piece assembly is simpler to manufacture but has a lower RF transparency.
An external antenna coupling RF energy through a shield typically must produce high RF currents. Depending on the shield design, to the extent RF transparency is reduced, antenna current, and thus voltage on the antenna, must be larger. Larger antenna current and voltage in turn leads to increased complexity of other parts of the RF circuit directly connected to the antenna, namely the tuning network and the RF connectors from the antenna to the tuning network. Further, since the antenna is positioned in air, the space surrounding the antenna is filled with RF flux, which is in essence unused flux, as it does not contribute to plasma generation. This comes at a price of increased antenna inductance, and thus increased antenna voltage.
The deposition shield must usually also be water-cooled to reduce its thermal cycling and particle shedding. In practice this may mean that a water connection is made in vacuum between the water feeds and the shield. While this is certainly feasible, it poses the risk of a water leak in the process module.
Further, for some applications, it is preferable to have the antenna positioned or wound around a cylindrical dielectric window or even around a frusto-conical dielectric window. Deposition shields for those applications are respectively cylindrical or conical. The machining of slots in such shields is very complex. This applies to soft-etch and pre-clean applications as well as deposition. However, these applications are usually performed at lower RF power and also at lower pressures. Under these conditions, design and manufacture of a deposition shield is simpler than for IPVD.
Accordingly, there is a need for the generation of dense and uniform plasma in the process volume with an antenna positioned inside the process volume.